The present invention relates to the general field of turbomachinery for aeronautical gas turbine engines. It is more specifically directed to the control of the clearance between the tips of mobile blades of a turbine rotor on the one hand, and a ring-shaped turbine shroud of an external housing surrounding the blades.
The existing clearance between the tip of the blades of a turbine and the ring-shaped shroud which surrounds them depends on the differences in dimensional variations between the rotating portions (disc and blades forming the turbine rotor) and fixed portions (external housing including the ring-shaped turbine shroud which it comprises). These dimensional variations have both a thermal origin (related to temperature variations of the blades, of the disc and of the housing) and a mechanical origin (notably related to the effect of the centrifugal force exerted on the turbine rotor).
In order to increase the performance of the turbine, it is desirable to minimize the clearance as much as possible. On the other hand, during an increase in speed, for example when passing from idling speed on the ground to take-off speed in a turbomachine for an aircraft engine, the centrifugal force exerted on the turbine rotor tends to bring the tips of blades closer to the ring-shaped turbine shroud before the ring-shaped turbine shroud has had time to expand under the effect of the increase in temperature related to the increase in speed. Therefore, there exists a contact risk in this operating point called a nipping point.
Resorting to active control systems is known for controlling the clearance of blade tips of a turbomachinery turbine. These systems generally operate by directing onto the outer surface of the ring-shaped turbine shroud, air for example taken up at a compressor and/or at the fan of the turbine engine. Fresh air sent over the outer surface of the turbine shroud has the effect of cooling the latter and thereby limiting its thermal expansion. Clearance is therefore minimized. Conversely, hot air promotes thermal expansion of the ring-shaped turbine shroud, which increases the clearance and for example gives the possibility of avoiding contact at the aforementioned nipping point.
Such an active control is controlled for example by the full authority digital engine control (or FADEC) system of the turbine engine. More specifically, the flow rate and/or the temperature of the air directed onto the ring-shaped turbine shroud is controlled depending on a set clearance and on an estimation of the actual blade tip clearance. Indeed, no sensor directly measures the blade tip clearance, which has therefore to be estimated.
As explained earlier, the blade tip clearance notably depends on the thermal state of the turbine engine. It is therefore known how to determine an estimation of the blade tip clearance, upon starting a turbine engine, depending on the thermal state of the turbine engine.
In certain engines, the thermal state upon starting is estimated according to the stoppage time of the engine. Recording the stoppage time of the engine is however a complex operation.
In certain engines, the turbine is equipped with a temperature sensor measuring the temperature of the housing, called Tcase. These are typically mass thermocouples (i.e. measuring the temperature of the material and not of the ambient air) of the K (chromel-alumel) type with a spring for compensating for the differential expansions and the vibrations. Such a sensor however has high cost, high mass and great bulkiness. Its installation at the housing, with its harness, is complex. Further, a loss of contact between the housing and the thermocouple may distort the measurement of the temperature.